1. Technical Field
This invention relates to a charge neutralizing system for neutralizing the static charge on sheets fed through a printing machine.
2. Background Art
Printing machines and especially those employing an electrophotographic printing process have employed various means to eliminate the static charge on printed sheets which pass through the machines. The electrophotographic process usually involves passing each sheet to a photoreceptor in the form of a photoconductor drum which bears a toned image which is to be transferred to the sheet. In order to effect the transfer of the toned image from the photoconductor drum to the sheet, a corona discharge device is utilized to charge the non-image receiving surface of the sheet to a polarity opposite that of the toner. The toner is thus attracted to the sheet which is then separated from the photoconductor drum. At this point, the charge on the sheet must be lessened in order to transport the sheet to a fixing station where the toner is heated so as to melt it and thus form or fix a permanent image on the sheet. Prior to fixing, the toned image may be disturbed as the charged sheet containing the toned image passes ground planes or the like as it progresses along the sheet transport. The charge on the sheets has typically been reduced by placing a second corona device in close proximity to the transfer station. This second corona device sprays the sheet with a charge opposite that sprayed at the transfer station. Such corona devices are connected to an AC or DC voltage source. It has further been necessary to utilize control circuits to closely regulate the amount of charge placed by the second corona device. Typical prior art patents disclosing such structure include U.S. Pat. Nos., 4,449,808, 4,640,606 and 4,688,927. Such power supplies and controls add further expense to the printing machine.
In addition to reducing the static charge of sheets at the transfer station and prior to the fixing station, prior electrophotographic printing machines have incorporated various other static elimination devices along the sheet feed path in order to facilitate the movement and stacking of the sheets. Such devices have included grounded brushes which either contact the sheet or which are located in close proximity to the sheet as disclosed in prior U.S. Pat. No. 4,494,166. Further, such prior art printing machines often provide several exit paper paths in order to selectively orient the sheet upon exit from the machine. Each such path requires static elimination for proper operation and stacking of the sheets.